Pijush K. Paul

Relationship between fractures, fault zones, stress, and reservoir productivity in the Suban gas field, Sumatra, Indonesia

It is becoming widely recognized that a relationship exists between stress, stress heterogeneity, and the permeability of subsurface fractures and faults. We present an analysis of the South Sumatra Suban gas field, developed mainly in fractured carbonate and crystalline basement, where active deformation has partitioned the reservoir into distinct structural and stress domains. These domains have differing geomechanical and structural attributes that control the permeability architecture of the field. The field is a composite of Paleogene extensional elements that have been modified by Neogene contraction to produce basement-rooted forced folds and neoformed thrusts. Reservoir-scale faults were interpreted in detail along the western flank of the field and reveal a classic oblique-compressional geometry. Bulk reservoir performance is governed by the local stress architecture that acts on existing faults and their fracture damage zones to alter their permeability and, hence, their access to distributed gas. Reservoir potential is most enhanced in areas that have large numbers of fractures with high ratios of shear to normal stress. This occurs in areas of the field that are in a strike-slip stress style. Comparatively, reservoir potential is lower in areas of the field that are in a thrust-fault stress style where fewer fractures with high shear-to-normal stress ratios exist. Achieving the highest well productivity relies on tapping into critically stressed faults and their associated fracture damage zones. Two wellbores have been drilled based on this concept, and each shows a three- to seven-fold improvement in flow potential.

Wellbore-Stability Study for the SAFOD Borehole Through the San Andreas Fault

This paper presents a wellbore-stability study of the San Andreas Fault Observatory at Depth (SAFOD) research borehole located near Parkfield, California, USA. In the summer of 2005, the SAFOD borehole was drilled successfully through the active trace of the San Andreas Fault (SAF) in an area characterized by fault creep and frequent microearthquakes. In this study, we report how the analysis of wellbore failures in the upper part of the hole, geophysical logs, and a model for stress gradients in the vicinity of the fault were used to estimate the mud weights required to drill through the fault successfully. Because logging-while-drilling (LWD) acoustic caliper data and real-time hole-volume calculations both showed that relatively little failure occurred while drilling through the SAF, the predicted mud weight was successful in drilling a stable borehole. However, a six-arm caliper log, run after drilling was completed, indicates that there was deterioration of the borehole with time, which appears to be caused by fluid penetration around the borehole. The LWD-resistivity measurements show that essentially no fluid penetration occurred as the hole was being drilled. Because of this, the mud weight used was capable of maintaining a stable wellbore. However, the resistivity data obtained after drilling show appreciable fluid penetration with time, thus negating the effectiveness of the mud weight and leading to time-dependent wellbore failure. Using finite-element modeling (FEM), we show that mud penetration into the fractured medium around the borehole causes failure with time.

A Method To Implement Permeability Anisotropy Associated With Fault Damage Zones in Reservoir Simulation

In this study, we present a method to incorporate the effects of fault damage zones (DZs) in a reservoir-simulation model. Permeability anisotropy associated with fault DZs depends on many factors, including the geometry of the faults in the reservoir and the associated dimension and density of fractures in the DZs. To model permeability anisotropy caused by fault DZs, we start by using geomechanically constrained discrete fracture models of DZs. Then, we use the orientation and size of the faults with reference to the grid axes to incorporate the effect of permeability anisotropy in the simulation grid. In this case study, in which faults are formed in an extensional regime, DZs show increased permeability along the strike of the fault and in the vertical direction, but there is no significant change in the permeability perpendicular to the faults. Inclusion of DZs in the simulation model shows significant improvement in the history matching in comparison to a base reservoir-simulation model with no DZs. Further, we analyze the uncertainty of the DZ modeling in the reservoir simulation by simulating multiple equiprobable models.